Abstract Solid solubility extension of Mg in Al was achieved for the binary Al–Mg system in the range of 10–40 at.% Mg by mechanical alloying of elemental powder mixtures and by mechanical milling of pre-alloyed ingots. No indication for the formation of the equilibrium phases (β-Al3Mg2 and γ-Al12Mg17) after milling was observed. The cell parameters of the solid solutions increase linearly with increasing Mg content with a slope that depends on the processing route used. During heating, the Al(Mg) solid solutions prepared from the different starting materials display a complex behavior characterized by several exothermic events. At low temperatures, an increasing amount of Mg is rejected from the solid solution with increasing temperature. At higher temperature, the β′-phase, a hexagonal intermediate phase with approximate composition Al3Mg2, is formed. The β′-phase most likely serves as precursor for the formation of the equilibrium β-Al3Mg2 phase, which occurs in the following exothermic event.